Reduce allocations

[inkel]

This PR reduces the allocations in getPattern and it also improves the overall speed of the function.

First note that I had to fix the existing benchmark, as the reader was being read only once in the first iteration, and afterwards it was always empty, thus parseCodeowners was parsing an empty file.

These were the metrics before the change:

$ go test -benchmem -bench=.
goos: darwin
goarch: arm64
pkg: github.com/hairyhenderson/go-codeowners
BenchmarkParseCodeowners-10      2677503               465.3 ns/op          4096 B/op          1 allocs/op
PASS
ok      github.com/hairyhenderson/go-codeowners 2.255s

After the change it's not showing more realistic data:

$ go test -benchmem -bench=.
goos: darwin
goarch: arm64
pkg: github.com/hairyhenderson/go-codeowners
BenchmarkParseCodeowners-10        31054             38910 ns/op           61424 B/op        641 allocs/op
PASS
ok      github.com/hairyhenderson/go-codeowners 3.529s

Then we move out of getPattern all regexp compilations, reducing the allocations ~62% and the performance in ~55%. Finally uses strings.ReplaceAll instead of the regexp versions when possible. These are the end results:

$ benchstat main.log stringsreplace.log
goos: darwin
goarch: arm64
pkg: github.com/hairyhenderson/go-codeowners
                   │  main.log   │         stringsreplace.log          │
                   │   sec/op    │   sec/op     vs base                │
ParseCodeowners-10   38.72µ ± 2%   15.21µ ± 7%  -60.73% (p=0.000 n=10)

                   │   main.log   │          stringsreplace.log          │
                   │     B/op     │     B/op      vs base                │
ParseCodeowners-10   59.95Ki ± 0%   25.98Ki ± 0%  -56.66% (p=0.000 n=10)

                   │  main.log  │         stringsreplace.log         │
                   │ allocs/op  │ allocs/op   vs base                │
ParseCodeowners-10   641.0 ± 0%   194.0 ± 0%  -69.73% (p=0.000 n=10)

[inkel]

I honestly don't know what the CI error is about 🤔

[hairyhenderson]

@inkel you'll need to amend the commit message(s) to match conventional commit patterns - maybe something like "fix(perf): Reduce allocations"

[inkel]

@hairyhenderson done!

[hairyhenderson]

Thanks for this @inkel!

[inkel]

Happy to contribute!